Internal combustion engines configured to operate with either liquid fuel, or a mixture of both liquid and gaseous fuels, are well known and increasingly used. There are many environments where engine power is desirable, and where gaseous fuel is more readily available or at a lower cost than liquid fuel. Examples include at certain landfills, wellheads, and some fuel transfer and distribution centers. In such applications it is often desirable to utilize the gaseous fuel for engine power as much as possible, relying on liquid fuel no more than necessary or practicable.
One known engine design within the above-mentioned general class employs an operator controlled mechanism for varying a relative amount of gaseous fuel in a mixture of gaseous and liquid fuels combusted within cylinders in the engine. In this design, a relatively small amount of liquid fuel will always be injected, but used at least predominantly as a pilot to ignite a main charge of gaseous fuel. The operator can adjust the relative amount of gaseous fuel in an attempt to optimize its use, for controlling fuel costs or for other reasons such as minimizing certain emissions. Such systems may work adequately, but have their drawbacks. Among these can be a need to be conservative as to the relative amount of gaseous fuel to avoid problems in operating the engine that might otherwise result.
U.S. Pat. No. 4,953,515 to Fehr et al. is directed to a diesel engine secondary fuel injection system, where a secondary fuel such as propane is injected into the engine in an amount which varies as a function of a monitored parameter. The monitored parameter is stated to include turbocharger boost pressure, fuel pressure in diesel fuel injector galleries, intake manifold pressure or combinations of such parameters. While Fehr may offer advantages, the principle purpose of the concept appears to be for performance enhancement, and the strategy is likely less applicable where different goals are sought.